1. Field of the Invention
The present invention relates to a zinc-electroplated steel sheet which is used for a wide range of applications, such as household electric appliances, automobiles and construction materials, and a method thereof.
2. Description of the Related Arts
Various types of chromate-treated zinc-electroplating are increasingly demanded to be used for zinc-electroplated steel sheets without having a coating thereon for application of household electric appliances, and thus become an important application field. Since they are used without having a coating thereon, it is required that they should present an excellent surface appearance. The primary condition to satisfy the excellent surface appearance is to be free of surface defects, such as nonuniform plating or the like. A high degree of whiteness is also an important condition. Further, the surface is usually coated after it has been subjected to a phosphate treatment, in which case, if pale-colored coating or a thin film is applied to a phosphate-treated original sheet having a low degree of whiteness, there is a deterioration in its image clarity after it has been coated. It is thus required that the original sheet have a high degree of whiteness. The outer appearance of the sheet which has been subjected to various types of the above-described conversion treatments is certainly determined by the outer appearance obtained after the sheet has been subjected to plating. It is, therefore, necessary that before being subjected to the conversion treatments, the zinc-plated steel sheet be uniform and have a high degree of whiteness.
There are two types of nonuniform plating of the zinc-plated steel sheets. One type is caused by defects inherent in plating equipment, while the other type arises from the surface defects of the original sheets. The former type of nonuniform plating can be improved by eliminating the defects of the plating equipment. The latter type of nonuniform plating should naturally be eliminated by improving the surface defects of the original sheets. However, it is difficult to completely remove the surface defects of the original sheets from the industrial point of view, and no actual measures taken against such problems have yet been reported.
Various methods have been proposed to improve the outer appearance of zinc-plated steel sheets, such as: an old method of improving a degree of glossiness by adding polyacrylamide or polyvinyl alcohol to a plating bath, as disclosed in Japanese Examined Patent Publication No. 46-38888; a method of obtaining the outer appearance without exhibiting glare by performing an oxidation treatment subsequent to a plating pretreatment, as disclosed in Japanese Patent Laid-Open No. 63-100193; and a method of performing plating evenly in a white color with a high current density by adding non-ionic polyacrylamide to an acid zinc-plating bath, as disclosed in Japanese Examined Patent Publication No. 01-36559. Among these publications, only Japanese Examined Patent Publication No. 01-36559 refers to a degree of whiteness. In this publication, however, an improvement in the degree of whiteness is achieved only when a high current density having a range of from 100 to 450 A/dm.sup.2 is applied, and if plating is performed with a current density lower than 100 A/dm.sup.2, with which current density plating is usually carried out, no improvement in a degree of whiteness is observed. Thus, it is difficult to put such a method into practical use. As is understood from the foregoing description, no zinc-electroplated steel sheets which are free from surface defects, such as the nonuniform plating, and also have a high degree of whiteness have yet been proposed.
A technique of adding tin to an acid pickling bath is disclosed in Japanese Examined Patent Publication No. 57-14758. However, as disclosed in the specification, an object of the invention of this publication is to inhibit decreases in the corrosion resistance and heat resistance when zinc-electroplating using insoluble lead electrodes is employed in which a small amount of lead eluted from the lead anode is codeposited on the plating so as to be thermally treated. However, there is no description of the outer appearance of plating whatsoever in this publication. Additionally, in this method, a steel sheet is used as a cathode in an acid pickling solution so as to perform electrolytic pickling, with the result that a large amount of deposited tin as much as from 10 to 40 mg/m.sup.2 is produced.
The present inventors made a study of the nonuniform plating caused by the surface defects of the original sheet, and found that the nonuniform plating arises from an extremely small amount of silica, alumina, titania and the like which are thickened on the surface of the original sheet. It was further understood that the zinc crystal deposited on the portion in which these oxides were thickened formed a finer-grained structure than that deposited on the portion free from these thickened oxides, which gives rise to a disparity in the outer appearance, thus making the nonuniform plating detectable. Although it is not clear what causes a finer-grained structure of the zinc crystal on the oxide-thickened portions, it is expected that the electrical resistance is increased in the oxide-thickened portions, which is considered to induce the reduction reaction of zinc ions and further to influence the generation of the crystalline nucleus, leading to the crystal growth. As has been discussed above, if the segregation of these oxides can be completely prevented, the nonuniform plating caused by the segregation will be avoided. However, only a very small amount of the oxides are segregated, and it is thus impossible to completely prevent them. The present inventors proceeded to make a study of a manufacturing method in which the nonuniform plating would be prevented even though a small amount of the oxides were segregated on the plating original sheet, and then, they found that an extremely small amount of tin was allowed to be deposited prior to plating, thereby preventing nonuniform plating. A mechanism in which nonuniform plating can be prevented is considered as follows.
Since tin is a much nobler metal than zinc and is also nobler than iron, it can be easily deposited. Accordingly, it can be readily deposited on both the surface containing the oxides and the surface not containing the oxides, which forms a uniform tin deposition layer, resulting in a uniform surface when zinc is deposited, thereby preventing the non-uniformity. Further, the present inventors proceeded to study an industrial and inexpensive application of a method of improving the nonuniform plating by means of the deposition of a small amount of tin. Then, they invented a nonuniform plating improvement method in which tin is added to an acid pickling solution, acid pickling being performed as a pretreatment of zinc plating. With this method, zinc-plated steel sheets can be manufactured without altering the currently-used zinc plating equipment and almost without increasing the manufacturing cost.